Left vs. Right Non-Inferiority Trial

Inclusion Criteria: 1. are female or male; 2. are outpatients; 3. are voluntary and competent to consent to treatment; 4. have a DSM 5 diagnosis of MDD,55 single or recurrent confirmed by Mini-International Neuropsychiatric Interview (MINI) version 7.0; 5. are 18yo to 65yo; 6. have failed to achieve a clinical response to an adequate dose of an antidepressant based on an Antidepressant Treatment History Form (ATHF) score of \> 3 in the current episode OR have been unable to tolerate at least two separate trials of antidepressants at less than the minimum adequate dose and/or duration (ATHF 1 or 2); 7. have a score ≥ 18 on the Hamilton Depression Rating Scale (HDRS-17 item); 8. have had no increase or initiation of any psychotropic medication in the 4 weeks prior to screening; 9. are able to adhere to the treatment schedule; 10. pass the TMS and MRI adult safety screening questionnaires. Exclusion Criteria: 1. have a history of substance use within the last 3 months; 2. have a concomitant major unstable medical illness; 3. have active suicidal intent; 4. are pregnant; 5. have a lifetime (MINI) diagnosis of any psychotic or bipolar disorder; 6. have a MINI anxiety disorder or personality disorder assessed by a study investigator to be primary and causing greater impairment than MDD; 7. have failed a course of ECT in the current episode; 8. have any significant neurological disorder, any history of seizure (except those therapeutically induced by ECT), significant head trauma with loss of consciousness for \> 5 min; 9. have any intracranial implant (e.g., aneurysm clips) or any other metal object within or near the head, excluding the mouth, that cannot be safely removed; 10. are participating in psychotherapy, must have been in stable treatment for at least 3 months prior to study entry, with no anticipated change in the frequency of therapeutic sessions, or focus of therapeutic sessions over the duration of the study; 11. have a clinically significant laboratory abnormality, in the opinion of the one of the principal investigators; 12. have a non-correctable clinically significant sensory impairment (i.e., cannot hear well enough to cooperate with interview);